AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 8 1 ~ 3 5 7 3 6 1(1990)
Longevity Factor in Hominoid Social Organization M.E. STEPHENS Department of Anthropology, University of Calgary, 2500 Uniuersity Drive N . W., Calgary, Alberta, Canada T2N IN4
KEY WORDS
Hominoidea, Incest, Descent theory, Patrilineality
ABSTRACT In most primate groups emigration of the maturing young of one or the other sex tends to serve as an incest avoidance mechanism. Among most primate species it is the males who change groups. This supports the theory that, in terms of reproductive success, males should compete for mates and females should compete for resources. In hominoids the combination of increased longevity and greater female discrimination in mate selection seems responsible for female emigration. This may relate to the high frequency of patrilocality and male control of resources among human groups. berg, 1979).Male hamadryas baboons (KumPast theorists in anthropology gave a mer, 1971, p. 17) and gorillas (Fossey, 1982) control harems of unrelated females. Faculgreat deal of thought to a false problem-the tative polyandry is found among saddleshift from matriarchies to patriarchies in human social evolution. Such a diachronic backed tamarins (Goldizen, 1987; Sussman shift in sexual control of resources no longer et al., 1987). Johann Bachofen and Lewis seems necessary in explanations of human Henry Morgan could have found examples of social groups. However, a review of modern their proposed stages of human social orgaliterature on primatology indicates that a nization among modern primates. Despite the variations, there is a pattern shift in sexual control of resources does occur synchronically across the primate spectrum to be discerned across the range of primate of social organization. Ethologists use terms social organization. Among the anthropoids, in a slightly different manner than do eth- monogamy or polyandry tends to be most nologists. Thus, “matriarchy” becomes “fe- common among New World monkeys. male-bonded group” and “patriarchy” or LLfra- Groupings based on female bonds are most ternal interest group” becomes “male- common among Old World monkeys, and bonded group.” %test" to an ethnologist is apes have no groups based on female bonds. When human mating patterns are considmating within a socially defined category, but this paper uses the term to mean a ered part of the primate spectrum an interspecific form of inbreeding involving hetero- esting transition may be noted across the sexual mating between primary kin (Moore, range of possibilities. The most common ba1984). Most species of anthropoid monkeys sis for non-human primate social organizahave female-bonded groups and most hu- tion is the female-bonded group. The most mans have male-bonded groups. This paper common basis for human social organization is an attempt t o explain that sexual shift in seems to be the fraternal interest group indicontrol of resources across the modern spec- cated by patrilocality. Richard Wrangham (1980) has proposed a trum of human and non-human primate somodel that explains the prevalence of fecial organization. male-bonded groups among non-human priBACKGROUND mates. A crude summary of a much more elegant theory is as follows. In terms of Studies of primate social organization and mating patterns indicate that a variety of possibilities are exploited by primate groups. These include separate-sex troops among Received September 1,1988; accepted March 29,1989. squirrel monkeys (Hrdy, 1981, p. 73) and A preliminaryversion of this paper, titled “JocastaWas Not a talapoins (Jolly, 1985, p. 253). Many New Chimpanzee,” was presented at the Northwest Anthropological World primates are monogamous (Eisen- Meetings,Tacoma, Washington,10 March 1988. INTRODUCTION
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reproductive success, the crucial limiting factor for females will be access to nutritional resources. The crucial limiting factor for males will be access to fertile females. Thus, females should be expected to arrange themselves in time and space in a manner that will allow them to exploit nutritional resources most efficiently. Males should be expected to deploy themselves to exploit the females most efficiently. The assumption that females should disperse to avoid competition with relatives loses validity in situations where the benefits of cooperation, calculated in terms of reproductive success, outweigh the costs of competition. If a key element of the animal’s diet is a rich, but patchily distributed, food then females will benefit by cooperating with related females to control this resource. Thus, the importance of ripe fruit in the diet of a primate is a good indicator of the importance of bonding with female relatives. The Wrangham model has been contested by van Schaik (19831, who feels that predation is a larger factor in sociality than diet and female competition. Both the van Schaik hypothesis and the Wrangham model are reconsidered by Jim Moore (1984))who concludes that the van Schaik data actually support the Wrangham model but that other factors should be considered with reference to folivorous colobines. This paper suggests that at least one other factor should be considered with reference to hominoids. Female-bonding is achieved by male emigration from the natal group. All mammal groups have some incest avoidance mechanism. In some mammal groups, females leave at maturity. In others, males or the maturing young of both sexes leave the natal group. The most common pattern in mammal groups is male emigration (Jolly, 1985, p. 124). Ripe fruit is an important dietary component for the majority of non-human primates. According to Wrangham’s model the distribution of tropical fruit means that male emigration should be common and femalebonded groups will be the result. This explains the predominance of female-bonded groups among non-human primates. The human situation is very different. The typical social organization found among humans is almost a polar opposite of that found among the majority of other primate species. In his book on matrilocality, Divale (1984, pp. 12, 14) notes that 70% of the world‘s
societies practice either patrilocal or virilocal residence and concludes that “virilocal and patrilocal residence are the basic human residence form.” He views other forms of residence among preliterate people as temporary aberrances caused by the stress of increased pressure on resources. Obviously, in this model, female emigration is the norm among human primates. There also seems to be a very basic difference in the control of resources between human and non-human primates. Robin Fox’s (1967, p. 31) book on human kinship systems lists four rules upon which all human kinship systems are based. Rule number three states that “The men usually exercise control.” In non-human primates, females tend to control nutritional resources. This is rarely the case among humans. Because humans, as mammals, are equally limited by constraints on reproductive success it would seem that access to nutritional resources must be as important to a human female as it is to a rhesus macaque. Is the distribution of human food very different? As Gaulin and Sailer (1985) have indicated, high-energy food sources are less abundant. Humans are large in size and consume a relatively high-energy diet. Such a diet tends to be patchy in its natural distribution. These factors would lead one to assume that, if the Wrangham model can be used for human primates, females should be forming groups of cooperating relatives to control such food resources. Human data indicate that the Wrangham model does not work for human social organization. Application of the Wrangham model to other species of hominoids may shed some light on the lack of fit with human primates. Alison Jolly (1985, p. vii) claims that “Primates stand at the hinge of evolution.” In that case, non-human hominoids are the hinge between humans and other primates, and chimpanzees are the pivotal point between human and non-human hominoids. An examination of the diet and social organization of non-human hominoids indicates that each of the four types of ape has a unique solution to its dietary and reproductive requirements. There are, however, two important similarities among all of the solutions. All species of apes have female emigration and no species is based on a core of related females. These points could also be made about the majority of humans.
LONGEVITY IN HOMINOID SOCIAL ORGANIZATION
The Hylobatidae are the least closely related to humans of all the hominoids. Seven species of gibbon and a single species of siamang make up the membership of the lesser apes. Ripe fruit is an important component of the diet of these animals. Each adult female gibbon or siamang shares a territory and the food resources within that territory with a single adult male and from one to four immature offspring. The mating pattern is one of fairly stable monogamy. At maturity both males and females emigrate from the territory of their parents. Chivers (1977) reports that, in some cases, a gibbon pair will help a son establish a territory adjacent to their own. Daughters are more widely dispersed. Tilson’s (1981) study of Kloss’s gibbons generally supports this view of social organization. He suggests that same-sex intolerance is the proximate cause of both male and female transfer. He also notes cases where gibbons have helped children of both sexes to establish territories in adjacent areas and one case of a male replacing a deceased father. Pongo pygmaeus, the orangutan, is the large arboreal ape native to Borneo and Sumatra. Orangutan social organization is often referred to as “solitary but social.” It seems to be limited by a combination of factors. Orangutans combine a patchily distributed diet of ripe fruit and leaf shoots with a large body and specialized mode of arboreal locomotion. Individual females share a home range with a single immature offspring. Males exploit larger ranges that overlap the ranges of females. Orangutans seem forced into a relatively solitary existence by the fact that a large area is required to satisfy the nutritional needs of a slow-moving animal. At maturity both males and females leave their mothers. Daughters may establish a range near their mothers (Galdikas, personal communication), but males disperse more widely. Both Galdikas (1985) and Rodman (1984)comment on the greater degree of transience in male orangutans. Neither one explicitly suggests a proximate cause of transfer, although the Rodman paper indicates that a patchily distributed diet, large body size, and slow mode of locomotion force maturing females to find a home range separate from their mothers. The African gorilla, Gorilla gorilla, is also a large ape. However, the gorilla moves on the ground and is able to digest mature leaves. This diet is low in energy but abundant and even in distribution. There is little
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reason for a female to cooperate o r to compete with other females in gaining access to this resource. Gorillas live in true harems. A single mature male is the focal point of a group that contains one or more females and their single immature offspring. Silverbacked males seem to differ individually in their tolerance of maturing males within the group. Some males manage to stay in their natal groups but most leave the group and live as solitaries until they acquire a harem or mate. Mature females eventually transfer to other harems or to solitary males. These transfers seem to be facilitated by the noise of gorilla copulations. The noise attracts other males or other harems, and non-estrous females tend to change groups during these times of propinquity (Fossey, 1982). Chimpanzees, Pan troglodytes and Pan paniscus, are the closest relatives of humans. They are also the non-human primate group that conforms least well to Wrangham’s model. The diet of chimpanzees is high in ripe fruit. Chimpanzees are large animals but the combination of their intelligence and their rapid mode of terrestrial locomotion enables them to exploit large territories as a group (Ghiglieri, 1985). Despite the diet and its distribution, chimpanzee social organization is based on a core of related males. These males seem to control nutritional resources as well as the females within the territory. Wrangham has an explanation for this discrepancy. According t o his theory, female chimpanzees are basically similar to female orangutans and each controls a small territory that is shared with her offspring. The male chimpanzee group controls a large territory that has females distributed through it “like raisins in a pudding.” He..sees the females as solitary but social and the males as a larger social unit. Both Ghiglieri and Goodall disagree with this interpretation (van Lawick-Goodall 1971; Goodall, 1986). They insist that both the males and the females use a large territory and interact with the total group. A fission-fusion mode of situationally dependent exploitation enables the group to control rich but patchily distributed food sources (Ghiglieri, 1985). Females leave the group a t maturity. Males remain within their natal group and seem to form fraternal interest groups that control both nutritional resources and females. DISCUSSION
The description of non-human hominoid social organization presents two related
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problems. Why do chimpanzees not fit them to resist incestuous matings more Wrangham’s model, and why is female emi- strongly than do males. Literature on hugration the norm among hominoids?There is man incest indicates that heterosexual incest is most common among father-daughter a single answer to both questions. Hominoids have a longer life span than do dyads and least common among mother-son most primates. Longevity data on long-lived, dyads (Shepher, 1983; Fox, 1967). Goodall free-ranging primates are difficult to obtain, (1986) and others (Tutin and McGinnis, but the life spans indicated for various spe- 1981) report similar frequencies among cies of primates by Napier and Napier (1985) chimpanzees. The sexual dimorphism found support this assumption. This longevity fac- in all hominoids except lesser apes gives tor has implications for incest avoidance fathers an advantage in disagreements with among hominoids. In most primate groups daughters. Although mature sons have the the emigration of males is an adequate same size advantage in disputes with mothmechanism for incest avoidance. It prevents ers, the long primate socialization period of mother-son and brother-sister mating in the maternal authority seems to gwe mothers natal group. This leaves only the problem of authority over mating disagreements. father-daughter mating within the daughter’s natal group. The combination of slow CONCLUSIONS maturation of daughters and brief male tenure in groups due either to rapid turnover of Early evolutionary theorists were incormales or death greatly reduces the probabil- rect in assumptions that incest was common ity of father-daughter matings. The male among animals and “primitive” humans. tenure in single male female-bonded groups They were, however, astute in some of their seems to be especially brief. Chism et al. speculations about the importance of kin(1984)report male tenure among patas mon- ship and control of resources in regard to the keys as varying from 2 days to 9 months. success of offspring. This emphasis on Hrdy (1981)calculates an average of 2 years’ “blood, sex, and property” seems equally tenure for males among Hanuman langurs. valid to modern cultural ecologists who use In multi-male groups, the Altmanns report the terms “inclusive fitness,” “reproductive that the demography among savannah ba- success,” and “access to resources” in a simiboons makes the probability of parent-child lar vein. mating unlikely even without male emigraWrangham’s theory emphasizes the fetion (Altmann and Altmann 1979). Obvi- male control of resources in different enviously, these males would rarely have oppor- ronments and the male control of reproductunities to mate with daughters. tion in the environment created by the The idea that female emigration is impor- female solution to control of resources. In the tant to reproductive success in long-lived thinking of early anthropologists, kinship mammals implies that females have a had to be based on the mother-child bond greater stake in incest avoidance. It also since it was the only clearly recognizable implies that father-daughter matings are relationship. Conscious kin recognition is more probable than mother son-matings. not stressed in the ethological model. The Both of these assumptions are supported by type of kin bond will be based on the emigrathe literature on incest. tion pattern of the group. The emigration Incest may reduce the reproductive suc- pattern is related to an incest avoidance cess of an individual by producing deleteri- mechanism that may well have another ous offspring. Since a male’s reproductive proximate cause but ultimately avoids inpotential is far higher than a female’s, an cest. incestuous mating does little to reduce a An incest tabu is conscious, cultural, and male’s total reproductive success. Females, human, but an incest avoidance mechanism particularly primate females, are limited in occurs among animals in nature. Although their reproductive potential by both the high either male or female emigration will preenergy costs and high time costs involved in vent inbreeding among short-lived mamthe processes of gestation, lactation, and mals and between siblings in long-lived speinfant rearing. When considered as a propor- cies, female emigration is the obvious incest tion of total reproductive potential, even a avoidance mechanism among primates of single deleterious mating is extremely costly great longevity. It may also be an explanation for the importance of fraternal interest for a female primate. The greater costs to females should cause groups and male control of nutritional re-
LONGEVITY IN HOMINOID SOCJAL ORGANIZATION
sources among the long-lived human primate. ACKNOWLEDGMENTS
The author is indebted to the many students who have asked relevant questions and to Dr. James Paterson for providing many of the answers. LITERATURE CITED Altmann SA, Altmann J (1979)Demographic constraints on behavior and social organization. In I S . Bernstein and E. Smith (eds.): Primate Ecology and Human Origins. New York: Garland. Chism J , Rowel1 T, and Olson D (1984) Life history patterns of female patas monkeys. In M. Small (ed.): Female Primates: Studied by Women Primatologists. New York: Alan R. Liss, Inc. Chivers DJ (1977) The lesser apes. In H.S.H. Prince Ranier I11 and G.H. Bourne (eds.): Primate Conservation. New York: Academic Press. Divale W (1984) Matrilocal Residence in Preliterate Society. Ann Arbor, Mich.: U.M.I. Research Press. Eisenberg JF (1979)Habitat, economy and society: Some correlations and hypotheses for the neotropical primates. In IS. Bernstein and E. Smith (eds.): Primate Ecology and Human Origins. New York: Garland. Fossey D (1982) Reproduction among free-living mountain gorillas. Am. J . Primatol. 2:97-104. Fox R (1967) Kinship and Marriage: An Anthropological Perspective. Baltimore: Penguin. Galdikas BMF (1985) Orangutan sociality at Tanjung Puting. Am. J . Primatol. 9:lOl-119. Gaulin S, and Sailer LD (1985)Are Females the Ecological Sex? Am. Anthropol. 87:111-119. Ghiglieri MP (1985) The social ecology of chimpanzees. Sci. Am. 252(6):102-113.
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Goldizen AW (1987) Facultative polyandry and the role of infant carrying in wild Saddle-Back Tamarins. Behav. Ecol. Sociobiol.20:99-109. Goodall J (1986)The Chimpanzees ofGombe: Patterns of Behaviour. Cambridge: Belnap Press of Harvard University. Hrdy SB (1981) The Woman That Never Evolved. Cambridge: Harvard University Press. Hrdy SB (1984) Female reproductive strategies. In M. Small (ed.): Female Primates: Studied by Women Primatologists. New York: Alan R. Liss, Inc. Jolly A (1985) The Evolution of Primate Behavior, 2nd Edition. New York: Macmillan. Kummer H (1971)Primate Societies: Group Techniques of Ecological Adaptation. Chicago: Aldine. Moore J (1984) Female transfer in primates. Int. J. Primatol. 5(6):537-589. Napier JR, and Napier PH (1985)The Natural History of the Primates. Cambridge: The M.I.T. Press. Rodman PS (1984)Foraging and social systems of orangutans and chimpanzees. In P.S. Rodman and J.G.H. Cant (eds.): Adaptations for Foraging in Nonhuman Primates. New York: Columbia University Press. Shepher J (1983) Incest: A Biosocial View. New York: Academic Press. Sussman RW, and Garber PA (1987) A new interpretation of the social organization and mating system of the Callitrichidae. Int. J. Primatol. 8f1):73-93. Tilson RL (1981) Family formation strategies of Kloss’s gibbon. Folia Primatol. 35:259-287. Tutin CEG, and McGinnis PR (1981)Chimpanzee reproduction in the wild. In C.E. Graham (ed.): Reproductive biology of the Great Apes. New York Academic Press. van Schaik (1983) Why are diurnal primates living in groups? Behaviour, 87:120-144. vanLawick-Goodall J (1971) In the Shadow ofMan. New York: Dell Publishing Co., Inc. Wrangham, RW (1980) An ecological model of femalebonded primate groups. Behaviour, 75:262-300.
Longevity Factor in Hominoid Social Organization M.E. STEPHENS Department of Anthropology, University of Calgary, 2500 Uniuersity Drive N . W., Calgary, Alberta, Canada T2N IN4
KEY WORDS
Hominoidea, Incest, Descent theory, Patrilineality
ABSTRACT In most primate groups emigration of the maturing young of one or the other sex tends to serve as an incest avoidance mechanism. Among most primate species it is the males who change groups. This supports the theory that, in terms of reproductive success, males should compete for mates and females should compete for resources. In hominoids the combination of increased longevity and greater female discrimination in mate selection seems responsible for female emigration. This may relate to the high frequency of patrilocality and male control of resources among human groups. berg, 1979).Male hamadryas baboons (KumPast theorists in anthropology gave a mer, 1971, p. 17) and gorillas (Fossey, 1982) control harems of unrelated females. Faculgreat deal of thought to a false problem-the tative polyandry is found among saddleshift from matriarchies to patriarchies in human social evolution. Such a diachronic backed tamarins (Goldizen, 1987; Sussman shift in sexual control of resources no longer et al., 1987). Johann Bachofen and Lewis seems necessary in explanations of human Henry Morgan could have found examples of social groups. However, a review of modern their proposed stages of human social orgaliterature on primatology indicates that a nization among modern primates. Despite the variations, there is a pattern shift in sexual control of resources does occur synchronically across the primate spectrum to be discerned across the range of primate of social organization. Ethologists use terms social organization. Among the anthropoids, in a slightly different manner than do eth- monogamy or polyandry tends to be most nologists. Thus, “matriarchy” becomes “fe- common among New World monkeys. male-bonded group” and “patriarchy” or LLfra- Groupings based on female bonds are most ternal interest group” becomes “male- common among Old World monkeys, and bonded group.” %test" to an ethnologist is apes have no groups based on female bonds. When human mating patterns are considmating within a socially defined category, but this paper uses the term to mean a ered part of the primate spectrum an interspecific form of inbreeding involving hetero- esting transition may be noted across the sexual mating between primary kin (Moore, range of possibilities. The most common ba1984). Most species of anthropoid monkeys sis for non-human primate social organizahave female-bonded groups and most hu- tion is the female-bonded group. The most mans have male-bonded groups. This paper common basis for human social organization is an attempt t o explain that sexual shift in seems to be the fraternal interest group indicontrol of resources across the modern spec- cated by patrilocality. Richard Wrangham (1980) has proposed a trum of human and non-human primate somodel that explains the prevalence of fecial organization. male-bonded groups among non-human priBACKGROUND mates. A crude summary of a much more elegant theory is as follows. In terms of Studies of primate social organization and mating patterns indicate that a variety of possibilities are exploited by primate groups. These include separate-sex troops among Received September 1,1988; accepted March 29,1989. squirrel monkeys (Hrdy, 1981, p. 73) and A preliminaryversion of this paper, titled “JocastaWas Not a talapoins (Jolly, 1985, p. 253). Many New Chimpanzee,” was presented at the Northwest Anthropological World primates are monogamous (Eisen- Meetings,Tacoma, Washington,10 March 1988. INTRODUCTION
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1990 WILEY-LISS, INC
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M.E.STEPHENS
reproductive success, the crucial limiting factor for females will be access to nutritional resources. The crucial limiting factor for males will be access to fertile females. Thus, females should be expected to arrange themselves in time and space in a manner that will allow them to exploit nutritional resources most efficiently. Males should be expected to deploy themselves to exploit the females most efficiently. The assumption that females should disperse to avoid competition with relatives loses validity in situations where the benefits of cooperation, calculated in terms of reproductive success, outweigh the costs of competition. If a key element of the animal’s diet is a rich, but patchily distributed, food then females will benefit by cooperating with related females to control this resource. Thus, the importance of ripe fruit in the diet of a primate is a good indicator of the importance of bonding with female relatives. The Wrangham model has been contested by van Schaik (19831, who feels that predation is a larger factor in sociality than diet and female competition. Both the van Schaik hypothesis and the Wrangham model are reconsidered by Jim Moore (1984))who concludes that the van Schaik data actually support the Wrangham model but that other factors should be considered with reference to folivorous colobines. This paper suggests that at least one other factor should be considered with reference to hominoids. Female-bonding is achieved by male emigration from the natal group. All mammal groups have some incest avoidance mechanism. In some mammal groups, females leave at maturity. In others, males or the maturing young of both sexes leave the natal group. The most common pattern in mammal groups is male emigration (Jolly, 1985, p. 124). Ripe fruit is an important dietary component for the majority of non-human primates. According to Wrangham’s model the distribution of tropical fruit means that male emigration should be common and femalebonded groups will be the result. This explains the predominance of female-bonded groups among non-human primates. The human situation is very different. The typical social organization found among humans is almost a polar opposite of that found among the majority of other primate species. In his book on matrilocality, Divale (1984, pp. 12, 14) notes that 70% of the world‘s
societies practice either patrilocal or virilocal residence and concludes that “virilocal and patrilocal residence are the basic human residence form.” He views other forms of residence among preliterate people as temporary aberrances caused by the stress of increased pressure on resources. Obviously, in this model, female emigration is the norm among human primates. There also seems to be a very basic difference in the control of resources between human and non-human primates. Robin Fox’s (1967, p. 31) book on human kinship systems lists four rules upon which all human kinship systems are based. Rule number three states that “The men usually exercise control.” In non-human primates, females tend to control nutritional resources. This is rarely the case among humans. Because humans, as mammals, are equally limited by constraints on reproductive success it would seem that access to nutritional resources must be as important to a human female as it is to a rhesus macaque. Is the distribution of human food very different? As Gaulin and Sailer (1985) have indicated, high-energy food sources are less abundant. Humans are large in size and consume a relatively high-energy diet. Such a diet tends to be patchy in its natural distribution. These factors would lead one to assume that, if the Wrangham model can be used for human primates, females should be forming groups of cooperating relatives to control such food resources. Human data indicate that the Wrangham model does not work for human social organization. Application of the Wrangham model to other species of hominoids may shed some light on the lack of fit with human primates. Alison Jolly (1985, p. vii) claims that “Primates stand at the hinge of evolution.” In that case, non-human hominoids are the hinge between humans and other primates, and chimpanzees are the pivotal point between human and non-human hominoids. An examination of the diet and social organization of non-human hominoids indicates that each of the four types of ape has a unique solution to its dietary and reproductive requirements. There are, however, two important similarities among all of the solutions. All species of apes have female emigration and no species is based on a core of related females. These points could also be made about the majority of humans.
LONGEVITY IN HOMINOID SOCIAL ORGANIZATION
The Hylobatidae are the least closely related to humans of all the hominoids. Seven species of gibbon and a single species of siamang make up the membership of the lesser apes. Ripe fruit is an important component of the diet of these animals. Each adult female gibbon or siamang shares a territory and the food resources within that territory with a single adult male and from one to four immature offspring. The mating pattern is one of fairly stable monogamy. At maturity both males and females emigrate from the territory of their parents. Chivers (1977) reports that, in some cases, a gibbon pair will help a son establish a territory adjacent to their own. Daughters are more widely dispersed. Tilson’s (1981) study of Kloss’s gibbons generally supports this view of social organization. He suggests that same-sex intolerance is the proximate cause of both male and female transfer. He also notes cases where gibbons have helped children of both sexes to establish territories in adjacent areas and one case of a male replacing a deceased father. Pongo pygmaeus, the orangutan, is the large arboreal ape native to Borneo and Sumatra. Orangutan social organization is often referred to as “solitary but social.” It seems to be limited by a combination of factors. Orangutans combine a patchily distributed diet of ripe fruit and leaf shoots with a large body and specialized mode of arboreal locomotion. Individual females share a home range with a single immature offspring. Males exploit larger ranges that overlap the ranges of females. Orangutans seem forced into a relatively solitary existence by the fact that a large area is required to satisfy the nutritional needs of a slow-moving animal. At maturity both males and females leave their mothers. Daughters may establish a range near their mothers (Galdikas, personal communication), but males disperse more widely. Both Galdikas (1985) and Rodman (1984)comment on the greater degree of transience in male orangutans. Neither one explicitly suggests a proximate cause of transfer, although the Rodman paper indicates that a patchily distributed diet, large body size, and slow mode of locomotion force maturing females to find a home range separate from their mothers. The African gorilla, Gorilla gorilla, is also a large ape. However, the gorilla moves on the ground and is able to digest mature leaves. This diet is low in energy but abundant and even in distribution. There is little
359
reason for a female to cooperate o r to compete with other females in gaining access to this resource. Gorillas live in true harems. A single mature male is the focal point of a group that contains one or more females and their single immature offspring. Silverbacked males seem to differ individually in their tolerance of maturing males within the group. Some males manage to stay in their natal groups but most leave the group and live as solitaries until they acquire a harem or mate. Mature females eventually transfer to other harems or to solitary males. These transfers seem to be facilitated by the noise of gorilla copulations. The noise attracts other males or other harems, and non-estrous females tend to change groups during these times of propinquity (Fossey, 1982). Chimpanzees, Pan troglodytes and Pan paniscus, are the closest relatives of humans. They are also the non-human primate group that conforms least well to Wrangham’s model. The diet of chimpanzees is high in ripe fruit. Chimpanzees are large animals but the combination of their intelligence and their rapid mode of terrestrial locomotion enables them to exploit large territories as a group (Ghiglieri, 1985). Despite the diet and its distribution, chimpanzee social organization is based on a core of related males. These males seem to control nutritional resources as well as the females within the territory. Wrangham has an explanation for this discrepancy. According t o his theory, female chimpanzees are basically similar to female orangutans and each controls a small territory that is shared with her offspring. The male chimpanzee group controls a large territory that has females distributed through it “like raisins in a pudding.” He..sees the females as solitary but social and the males as a larger social unit. Both Ghiglieri and Goodall disagree with this interpretation (van Lawick-Goodall 1971; Goodall, 1986). They insist that both the males and the females use a large territory and interact with the total group. A fission-fusion mode of situationally dependent exploitation enables the group to control rich but patchily distributed food sources (Ghiglieri, 1985). Females leave the group a t maturity. Males remain within their natal group and seem to form fraternal interest groups that control both nutritional resources and females. DISCUSSION
The description of non-human hominoid social organization presents two related
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problems. Why do chimpanzees not fit them to resist incestuous matings more Wrangham’s model, and why is female emi- strongly than do males. Literature on hugration the norm among hominoids?There is man incest indicates that heterosexual incest is most common among father-daughter a single answer to both questions. Hominoids have a longer life span than do dyads and least common among mother-son most primates. Longevity data on long-lived, dyads (Shepher, 1983; Fox, 1967). Goodall free-ranging primates are difficult to obtain, (1986) and others (Tutin and McGinnis, but the life spans indicated for various spe- 1981) report similar frequencies among cies of primates by Napier and Napier (1985) chimpanzees. The sexual dimorphism found support this assumption. This longevity fac- in all hominoids except lesser apes gives tor has implications for incest avoidance fathers an advantage in disagreements with among hominoids. In most primate groups daughters. Although mature sons have the the emigration of males is an adequate same size advantage in disputes with mothmechanism for incest avoidance. It prevents ers, the long primate socialization period of mother-son and brother-sister mating in the maternal authority seems to gwe mothers natal group. This leaves only the problem of authority over mating disagreements. father-daughter mating within the daughter’s natal group. The combination of slow CONCLUSIONS maturation of daughters and brief male tenure in groups due either to rapid turnover of Early evolutionary theorists were incormales or death greatly reduces the probabil- rect in assumptions that incest was common ity of father-daughter matings. The male among animals and “primitive” humans. tenure in single male female-bonded groups They were, however, astute in some of their seems to be especially brief. Chism et al. speculations about the importance of kin(1984)report male tenure among patas mon- ship and control of resources in regard to the keys as varying from 2 days to 9 months. success of offspring. This emphasis on Hrdy (1981)calculates an average of 2 years’ “blood, sex, and property” seems equally tenure for males among Hanuman langurs. valid to modern cultural ecologists who use In multi-male groups, the Altmanns report the terms “inclusive fitness,” “reproductive that the demography among savannah ba- success,” and “access to resources” in a simiboons makes the probability of parent-child lar vein. mating unlikely even without male emigraWrangham’s theory emphasizes the fetion (Altmann and Altmann 1979). Obvi- male control of resources in different enviously, these males would rarely have oppor- ronments and the male control of reproductunities to mate with daughters. tion in the environment created by the The idea that female emigration is impor- female solution to control of resources. In the tant to reproductive success in long-lived thinking of early anthropologists, kinship mammals implies that females have a had to be based on the mother-child bond greater stake in incest avoidance. It also since it was the only clearly recognizable implies that father-daughter matings are relationship. Conscious kin recognition is more probable than mother son-matings. not stressed in the ethological model. The Both of these assumptions are supported by type of kin bond will be based on the emigrathe literature on incest. tion pattern of the group. The emigration Incest may reduce the reproductive suc- pattern is related to an incest avoidance cess of an individual by producing deleteri- mechanism that may well have another ous offspring. Since a male’s reproductive proximate cause but ultimately avoids inpotential is far higher than a female’s, an cest. incestuous mating does little to reduce a An incest tabu is conscious, cultural, and male’s total reproductive success. Females, human, but an incest avoidance mechanism particularly primate females, are limited in occurs among animals in nature. Although their reproductive potential by both the high either male or female emigration will preenergy costs and high time costs involved in vent inbreeding among short-lived mamthe processes of gestation, lactation, and mals and between siblings in long-lived speinfant rearing. When considered as a propor- cies, female emigration is the obvious incest tion of total reproductive potential, even a avoidance mechanism among primates of single deleterious mating is extremely costly great longevity. It may also be an explanation for the importance of fraternal interest for a female primate. The greater costs to females should cause groups and male control of nutritional re-
LONGEVITY IN HOMINOID SOCJAL ORGANIZATION
sources among the long-lived human primate. ACKNOWLEDGMENTS
The author is indebted to the many students who have asked relevant questions and to Dr. James Paterson for providing many of the answers. LITERATURE CITED Altmann SA, Altmann J (1979)Demographic constraints on behavior and social organization. In I S . Bernstein and E. Smith (eds.): Primate Ecology and Human Origins. New York: Garland. Chism J , Rowel1 T, and Olson D (1984) Life history patterns of female patas monkeys. In M. Small (ed.): Female Primates: Studied by Women Primatologists. New York: Alan R. Liss, Inc. Chivers DJ (1977) The lesser apes. In H.S.H. Prince Ranier I11 and G.H. Bourne (eds.): Primate Conservation. New York: Academic Press. Divale W (1984) Matrilocal Residence in Preliterate Society. Ann Arbor, Mich.: U.M.I. Research Press. Eisenberg JF (1979)Habitat, economy and society: Some correlations and hypotheses for the neotropical primates. In IS. Bernstein and E. Smith (eds.): Primate Ecology and Human Origins. New York: Garland. Fossey D (1982) Reproduction among free-living mountain gorillas. Am. J . Primatol. 2:97-104. Fox R (1967) Kinship and Marriage: An Anthropological Perspective. Baltimore: Penguin. Galdikas BMF (1985) Orangutan sociality at Tanjung Puting. Am. J . Primatol. 9:lOl-119. Gaulin S, and Sailer LD (1985)Are Females the Ecological Sex? Am. Anthropol. 87:111-119. Ghiglieri MP (1985) The social ecology of chimpanzees. Sci. Am. 252(6):102-113.
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Goldizen AW (1987) Facultative polyandry and the role of infant carrying in wild Saddle-Back Tamarins. Behav. Ecol. Sociobiol.20:99-109. Goodall J (1986)The Chimpanzees ofGombe: Patterns of Behaviour. Cambridge: Belnap Press of Harvard University. Hrdy SB (1981) The Woman That Never Evolved. Cambridge: Harvard University Press. Hrdy SB (1984) Female reproductive strategies. In M. Small (ed.): Female Primates: Studied by Women Primatologists. New York: Alan R. Liss, Inc. Jolly A (1985) The Evolution of Primate Behavior, 2nd Edition. New York: Macmillan. Kummer H (1971)Primate Societies: Group Techniques of Ecological Adaptation. Chicago: Aldine. Moore J (1984) Female transfer in primates. Int. J. Primatol. 5(6):537-589. Napier JR, and Napier PH (1985)The Natural History of the Primates. Cambridge: The M.I.T. Press. Rodman PS (1984)Foraging and social systems of orangutans and chimpanzees. In P.S. Rodman and J.G.H. Cant (eds.): Adaptations for Foraging in Nonhuman Primates. New York: Columbia University Press. Shepher J (1983) Incest: A Biosocial View. New York: Academic Press. Sussman RW, and Garber PA (1987) A new interpretation of the social organization and mating system of the Callitrichidae. Int. J. Primatol. 8f1):73-93. Tilson RL (1981) Family formation strategies of Kloss’s gibbon. Folia Primatol. 35:259-287. Tutin CEG, and McGinnis PR (1981)Chimpanzee reproduction in the wild. In C.E. Graham (ed.): Reproductive biology of the Great Apes. New York Academic Press. van Schaik (1983) Why are diurnal primates living in groups? Behaviour, 87:120-144. vanLawick-Goodall J (1971) In the Shadow ofMan. New York: Dell Publishing Co., Inc. Wrangham, RW (1980) An ecological model of femalebonded primate groups. Behaviour, 75:262-300.
